House leaders decided Saturday to stage a vote on the Senate’s health-care bill, dropping a much-criticized strategy of allowing lawmakers to “deem” the landmark legislation into law. But the outcome of that vote remained in doubt as a pivotal bloc of Democrats continued to withhold its support over fears that the bill would open the door to the federal funding of abortion.

House leaders were working to secure their votes late Saturday with the promise of an executive order affirming President Obama’s commitment to a longstanding ban on public abortion funding except in cases of rape or incest, or to save the life of the mother. Rep. Marcy Kaptur (D-Ohio), a key antiabortion vote, said she thought the document would be insufficient to bring the entire group of about 10 antiabortion Democrats onboard.

Senior Democrats predicted a cliffhanger when the House is expected to vote Sunday night, saying they are likely to clear the 216-vote threshold for final passage by the narrowest of margins. Democratic leaders huddled in the office of House Speaker Nancy Pelosi (D-Calif.) late into the evening, reviewing the final list of commitments.

Throughout the day, thousands of angry protesters milled outside the Capitol; some hurled insults at black and gay lawmakers and shouted at Democrats to “kill the bill!” Meanwhile, Obama made a final pitch for reform, exhorting wavering lawmakers to rise to the aid of millions of uninsured Americans by taking “the single most important step that we have taken on health care since Medicare” was created in 1965.